With automatic transmissions for motor vehicles, such as is known (for example) from DE 10 2005 002 337 A1, the transmission ratio stages are adjusted by means of shifting elements, also referred to as clutches. Clutches may connect two rotatable elements together, and one element connected in a torque-proof manner to the transmission housing to one rotatable element. In the latter case, one also speaks of a brake. Thereby, the shifting elements are formed as frictional-locking multi-disk clutches and positive-locking shifting elements, such as claw couplings (i.e., dog clutches). In order to carry out power shifts, i.e. a change to the transmission ratio without an interruption of the pulling force, at least one part of the shifting elements must be formed as frictional-locking shifting elements.
With known automatic transmissions, actuation (i.e. the locking of the shifting elements for the transfer of torque) takes place hydraulically, i.e. by means of actuators in the form of piston-cylinder units, which are subjected to a pressure medium, typically transmission oil. A clutch pressure chamber is formed from the piston and the cylinder; for the actuation of the shifting element, this is subjected to a pressure oil under a control pressure. The pressure oil is conveyed by a motor-driven pump as a pressure source and, in particular for frictional-locking shifting elements during their entire duration of actuation, must be kept at a pressure level that produces pressing force of the multi-disks in the shifting element that is sufficient for the transfer of torque. The energy to generate pressure by means of the hydraulic pump must be applied by the engine of the motor vehicle, which has effects on fuel consumption and CO2 emissions. Due to the power losses, the energy available for the drive of the vehicle is reduced, by which the transmission efficiency of the drive train is lowered.
In addition, leakage losses arise at sealing points, such as pressure oil supply lines from the transmission housing through so-called “rotary oil supply lines,” which are sealed by means of gap seals, such as slide bearings and/or rectangular rings, in the rotating transmission shaft. This requires an ongoing tracking of the oil pressure in the actuator with a locked shifting element or a replenishment of the leakage amount, in order to keep the shifting element locked.
In order to make the pressure in the actuator of the clutches independent of the supply pressure generated by the pump, and to keep the leakage losses to a minimum, the clutch pressure chamber can be sealed by a so-called “stop valve,” such that the clutch pressure prevailing therein is maintained without additional oil having to be replenished. Only during the shifting process is the valve opened and then filled with the corresponding pressure. A hydraulic control device with a stop valve is known from DE 102 05 411 A1. The supply pressure to be generated by the transmission pump can be lowered with respect to the clutch pressure trapped in the shifting element, by which the power consumption of the transmission oil pump, which is calculated as the product of the conveyed volume flow and the generated pressure difference, is significantly reduced. With the lower power consumption of the transmission pump, the overall efficiency of the transmission increases, since less engine power has to be branched off for the hydraulics as reactive power, and fuel consumption is reduced for the same performance. The stop valve known from DE 102 05 411 A1, without being subject to pressure, remains in the shut-off position, thus even at the standstill of the pump or the motor that drives the pump. Such a functioning of a stop valve, which remains closed even if the hydraulic system is pressureless (i.e. is at the level of the ambient pressure), is referred to as “normally closed”. Hereinafter, a hydraulic system that is under ambient pressure and is not subjected to a pressure generator such as a pump is referred to as “pressureless”.
In terms of functional reliability, it is thereby disadvantageous that the stop valve, and thus the relevant shifting element, can no longer be opened upon the stop or failure of the engine or the pump, and thus a lack of pressurization of the automatic transmission, such that a malfunction that blocks the drive train may arise.
The functioning of a stop valve, which is open upon the standstill of the pump or a pressureless system typically under the action of a spring, by which the clutch pressure degrades to ambient pressure, is referred to as “normally opened”. This functioning offers a major advantage in terms of functional reliability of a transmission, since, upon a failure of the hydraulic pressure supply, the power flow in the transmission is interrupted. However, in a disadvantageous manner, the stop valve must be constantly subjected to a control pressure in order to keep it closed. With a suitable design of the stop valve, particularly the selection of surfaces of a closing piston subjected to control pressure and clutch pressure, the control pressure may be significantly lower than the clutch pressure.
DE 102013221038.8, which was not pre-published, discloses a stop valve that is configured as “normally opened.” This essentially comprises a cylindrical closing piston, which is referred to as a so-called “seat piston” if the stop valve is formed as a seat valve. In this case, the closing piston is subjected from one side by the clutch pressure and from the other side by the control pressure. Due to the ratios of the pressurized surfaces of the closing piston, the control pressure, and thus at least the pressure generated by the pump, is significantly lower than the clutch pressure. With a pressureless hydraulic system, under the action of a compression spring, the stop valve opens the clutch pressure chamber for the remaining pressureless hydraulic system.
DE102014218581.4, which is also not pre-published, shows a possible embodiment of a “normally opened” stop valve, with which the control pressure can be further reduced compared to the trapped clutch pressure, as the clutch pressure chamber is not closed by the closing piston itself, but by means of a ball as a closing body, which is pressed by the closing piston over a ramp against a valve seat in a clutch channel to the clutch pressure chamber. Thereby, several balls can be distributed around the circumference. The closing piston is ring-shaped or formed in the shape of a hollow cylinder, and is arranged around a shaft concentric to it. Further, the ring-shaped closing piston is arranged radially inside of a shifting element. The radial expansion of the closing piston is relatively low, such that this embodiment is an arrangement with a low need for radial installation space, and does not require an additional axial structural length. The ring-shaped surface subjected to the control pressure is solely opposed to the relatively small projection surfaces of the balls subject to the clutch pressure, such that, compared to a stop valve designed according to DE102013221038.8, the surface subjected to the clutch pressure is significantly smaller than the space subject to the control pressure. However, the limit of the reduction of the surfaces is specified by a minimum value of the flow cross-section of the channels closed by means of the balls, below which the shifting element is not able to be filled quickly enough.
In addition, the full amount of the clutch pressure does not counteract the control pressure; rather, based on the application of force by the ball over a conical, chamfer-shaped ball ramp, which is formed on the piston, only one axial force component of the clutch pressure counteracts it. Thereby, it is possible to further lower the control pressure for closing the stop valve and thus the power consumption of the pump.